A positron CT apparatus, i.e., a PET (Positron Emission Tomography) apparatus reconstructs a tomographic image of a subject only upon simultaneous detection (i.e., coincidence counting) of two gamma-rays generated through annihilation of a positive electron (Positron) with a plurality of detectors. Specifically, the radiopharmaceutical containing positron-emitting radionuclide is administered to the subject. Then pair annihilation gamma-rays of 511 Kev emitted from the administered subject are detected with detectors each formed by groups of many detector elements (e.g., scintillators). Thereafter, when gamma-rays are detected with two detectors within a certain time period, the gamma-rays are detected “coincidently”, and are counted as pair annihilation gamma-rays. Moreover, a line (LOR: Line Of Response) is identified that connects the two detectors that detect positions where the pair annihilation gamma-rays are generated. Then information on coincidence that is detected in such a manner is stored for reconstruction, whereby a positron-emitting radionuclide image (i.e., a tomographic image) is obtained.
In recent years, such an apparatus has a trend to conduct a diagnosis by superimposing the tomographic image obtained by the PET apparatus on a shape image obtained by another type of modality apparatus. Accordingly, the number of apparatus such as a PET/CT apparatus has increased in which a plurality of PET apparatus and modality apparatus accept a mount table (e.g., bed) supporting the subject placed thereon.
However, addition of a new modality apparatus to the existing modality apparatus is difficult in terms of constraint of the bed. As a result, new purchase of a plurality of PET apparatus or modality apparatus is required. Accordingly, the present Applicant has suggested another type of PET apparatus. See Japanese Patent No. 4650324. That is, the PET apparatus is configured such that detector units and holding arms holding the detector units are mounted on a transportable carriage, and the detector units are disposed so as to surround the bed.
With the PET apparatus, the bed of the existing modality apparatus (e.g., X-ray CT apparatus) is commonly used and the carriage is transported close to the modality apparatus. This allows arrangement of the detector units adjacent to the modality apparatus. Consequently, additional arrangement of the PET apparatus to the existing modality apparatus is performable without new purchase of a plurality of PET apparatus and modality apparatus.
As illustrated in FIGS. 11(a) and 11(b), the PET apparatus includes two holding arms A1, A2 (a first C-shape arm, and a second C-shape arm) in an arc shape. The holding arms include at tips thereof detector units U1, U2 (a first gamma-ray detector, and second gamma-ray detector), respectively. The two holding arms A1, A2 move independently along a circle surrounding the subject M, thereby changing inclination angles of the detector units U1, U2 such that surfaces of the detector units U1, U2 into which gamma-rays enter face to each other across the subject M. Such a configuration allows variation in distance between the detector units U1, U2 depending on a size of the subject M while the inclination angles are changed as illustrated in FIGS. 11(a) and 11(b). This achieves radiography while the detector units U1, U2 face to each other.
[Patent Literature 1] Japanese Patent No. 4650324